1. Field of the Invention
The present invention relates to a high-frequency electronic device in which a high-frequency electronic circuit constituted by a line conductor is fabricated on a single-crystal dielectric substrate.
2. Description of the Related Art
A high-frequency electronic device in which a high-frequency electronic circuit constituted by a line conductor made of a thin-film conductor layer is fabricated on a single-crystal dielectric substrate, which device is used as a high-frequency component in a high-frequency electronic apparatus, has such a structure as shown by an exploded perspective view of FIG. 5A and a sectional view of FIG. 5B conventionally.
In FIGS. 5A and 5B, a high-frequency electronic circuit 2 constituted by a wiring conductor layer of a line conductor or the like is formed on the top face of a single-crystal dielectric substrate 1, and a ground conductor layer (ground plane) 3 is formed on the same face as or the opposite face to the high-frequency electronic circuit 2. This single-crystal dielectric substrate 1 is installed in a metal housing 4 and covered with a metal lid 5 which is attached to the top face of the metal housing 4, thereby hermetically stored in a container which is constructed of the metal housing 4 and the metal lid 5. Input and output of electric signals between the high-frequency electronic circuit 2 in the metal housing 4 and an outside is carried out via a connector 6 which is built in a side wall of the metal housing 4.
With regard to a wiring conductor layer which constitutes the high-frequency electronic circuit 2, other than a microstrip line structure of placing a line conductor on the top face of the single-crystal dielectric substrate 1 and placing the ground conductor layer 3 on the bottom face thereof as shown in FIGS. 5A and 5B, a coplanar line structure of placing a line conductor and a ground conductor layer so as to be parallel to the line conductor on the top face of a dielectric substrate, has been used. In these cases, an electromagnetic wave is radiated from the high-frequency electronic circuit 2 of the microstrip line structure or the coplanar line structure into the upper space of the line conductor formed on the single-crystal dielectric substrate 1, and such radiation of an electromagnetic wave causes degradation of the performance of the high-frequency electronic circuit 2 and an adverse effect on an external electronic circuit, so that it is necessary to make the high-frequency electronic circuit 2 have a structure of trapping an electromagnetic wave by a conductive member. For this reason, conventionally, a shield structure of covering the high-frequency electronic circuit 2 with the metal housing 4 and the metal lid 5 is adopted, whereby an electromagnetic wave is prevented from being radiated to the outside of the electronic device.
In such a conventional high-frequency electronic device, the high-frequency electronic circuit 2 is constituted by a line conductor of the microstrip line structure or the coplanar line structure as described above, and a line conductor of a strip line structure is not used, in which ground planes are respectively placed on the top and bottom faces of a dielectric substrate and a line conductor is formed as an internal wiring layer within the dielectric substrate interposed between these ground planes. The reason is that in the strip line structure, while it is necessary to dispose a through conductor for connecting the internal wiring layer with the signal line of an external circuit, in the case of using the single-crystal dielectric substrate as a dielectric substrate in order to cause a conductive material which constitutes the internal wiring layer to reach an orientational growth or a single-crystal growth, improve the flow of electricity, and reduce loss at the internal wiring layer, it has been impossible to produce the through conductor on a single-crystal dielectric substrate.
That is to say, a single-crystal dielectric substrate is produced by recrystallizing a raw material which is once fused, and hence it is impossible to process a through hole for disposing a through conductor in the producing step, so that there has been a problem that a line conductor which is formed inside the dielectric substrate cannot be connected with an external circuit via a through conductor.
Moreover, although it is possible to open a through hole on a single-crystal dielectric substrate by using a drill and the like, there has been a problem that the crystal structure around the through hole is disturbed when the through hole is thus opened, with the result that the line conductor cannot reach an orientational growth or a single-crystal growth in an excellent manner.
For these reasons, the strip line structure which hardly brings an adverse effect on the high-frequency electronic circuit and has excellent electric characteristics because the electromagnetic radiation coming from the line conductor is shielded down by the ground planes placed thereon and thereunder, has not been adopted in the conventional high-frequency electronic device.
On the other hand, in a configuration of the conventional microstrip line structure or coplanar line structure as described above, an electronic circuit 2 is covered with a metal housing 4 and a metal lid S in order to prevent electromagnetic radiation, and the metal housing 4 is big and heavy, so that there has been a problem that it is difficult to reduce the size and weight of a high-frequency circuit component. Besides, the upper space of the line conductor of the microstrip line structure or the coplanar line structure is hollow inside a container which is constructed of the metal housing 4 and the metal lid 5, and hence the efficiency of dissipating heat which is generated from the line conductor is low, so that there has also been a problem that the high-frequency electronic circuit is degraded in electric characteristics due to elevation in temperature or generation of a temperature distribution therein.